cmd/upspin: in 'setupserver' use ServerConfig when writing server config
cmd/upspin: in 'setupserver' use ServerConfig when writing server config

The code in setupserver assumed that the server user was upspin@domain,
which is what setupdomain always produces, but really we should trust
the serverconfig.json file to tell us the server user they're using.

I stumbled across this while trying to do some custom server deployments
using 'upspinserver' and 'upspin setupserver'.

Change-Id: I8004a800d48b27758bdd0f24b03a006911ebec9b
Reviewed-on: https://upspin-review.googlesource.com/8011
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin